Pixelated LED array with optical elements

ABSTRACT

The pcLED pixels in a phosphor-converted LED array each comprise an optical element on the light-emitting surface above the phosphor layer. In methods for making such pixelated LED arrays, a thin layer of a sacrificial phosphor carrier substrate is retained as the optical element on the output surface of the phosphor pixels upon completion of the fabrication process.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit of priority to U.S. Provisional PatentApplication No. 62/787,006 titled “Pixelated LED Array With OpticalElements” and filed Dec. 31, 2018, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The invention relates generally to phosphor-converted light emittingdiodes.

BACKGROUND

Semiconductor light emitting diodes and laser diodes (collectivelyreferred to herein as “LEDs”) are among the most efficient light sourcescurrently available. The emission spectrum of an LED typically exhibitsa single narrow peak at a wavelength determined by the structure of thedevice and by the composition of the semiconductor materials from whichit is constructed. By suitable choice of device structure and materialsystem, LEDs may be designed to operate at ultraviolet, visible, orinfrared wavelengths.

LEDs may be combined with one or more wavelength converting materials(generally referred to herein as “phosphors”) that absorb light emittedby the LED and in response emit light of a longer wavelength. For suchphosphor-converted LEDs (“pcLEDs”), the fraction of the light emitted bythe LED that is absorbed by the phosphors depends on the amount ofphosphor material in the optical path of the light emitted by the LED,for example on the concentration of phosphor material in a phosphorlayer disposed on or around the LED and the thickness of the layer.

Phosphor-converted LEDs may be designed so that all of the light emittedby the LED is absorbed by one or more phosphors, in which case theemission from the pcLED is entirely from the phosphors. In such casesthe phosphor may be selected, for example, to emit light in a narrowspectral region that is not efficiently generated directly by an LED.

Alternatively, pcLEDs may be designed so that only a portion of thelight emitted by the LED is absorbed by the phosphors, in which case theemission from the pcLED is a mixture of light emitted by the LED andlight emitted by the phosphors. By suitable choice of LED, phosphors,and phosphor composition, such a pcLED may be designed to emit, forexample, white light having a desired color temperature and desiredcolor-rendering properties.

SUMMARY

This specification discloses phosphor-converted LED arrays in which thepcLED pixels each comprise an optical element on the light-emittingsurface above the phosphor layer, as well as methods for making sucharrays. In the disclosed methods, a thin layer of a sacrificial phosphorcarrier substrate is retained as the optical element on the outputsurface of the phosphor pixels upon completion of the fabricationprocess.

This remaining thin layer of the phosphor carrier substrate allows formore tolerance in end point detection during removal of the sacrificialcarrier substrate without impacting optical properties of the pcLED suchas color point, for example. This can result in higher manufacturingyield.

Further, the carrier substrate material and the texture (finish) andgeometry of the carrier substrate surface can be used to affect theappearance and output of the pcLED in a desired manner. The carriersubstrate material may be, for example, a transparent glass, atransparent crystal, a tinted glass, or a translucent material. Theportion of the phosphor carrier substrate retained on the phosphor pixelmay be patterned or otherwise configured or selected to function, forexample, as a micro or meta-lens, a light extraction element, a dichroicfilter, an off-state white diffuser layer, a polarizer, or as any othersuitable optical element.

The retained portion of the sacrificial carrier substrate may have athickness of, for example, about 5 microns to about 50 micronsperpendicular to the light-emitting surface on the phosphor layer.

Other embodiments, features and advantages of the present invention willbecome more apparent to those skilled in the art when taken withreference to the following more detailed description of the invention inconjunction with the accompanying drawings that are first brieflydescribed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic cross-sectional view of an example pcLED.

FIGS. 2A and 2B show, respectively, cross-sectional and top schematicviews of an array of pcLEDs.

FIG. 3A shows a schematic top view of an electronics board on which anarray of pcLEDs may be mounted, and FIG. 3B similarly shows an array ofpcLEDs mounted on the electronic board of FIG. 3A.

FIG. 4A shows a schematic cross sectional view of an array of pcLEDsarranged with respect to waveguides and a projection lens. FIG. 4B showsan arrangement similar to that of FIG. 4A, without the waveguides.

FIG. 5A, FIG. 5B, FIG. 5C, FIG. 5D, FIG. 5E, and FIG. 5F are partialcross-sectional views that schematically show stages in an examplemethod for fabricating a pixelated pcLED array in which the pcLED pixelseach comprise an optical element on the light emitting surface above thephosphor pixel, formed from a retained portion of a sacrificial phosphorcarrier substrate.

DETAILED DESCRIPTION

The following detailed description should be read with reference to thedrawings, in which identical reference numbers refer to like elementsthroughout the different figures. The drawings, which are notnecessarily to scale, depict selective embodiments and are not intendedto limit the scope of the invention. The detailed descriptionillustrates by way of example, not by way of limitation, the principlesof the invention.

FIG. 1 shows an example of an individual pcLED 100 comprising asemiconductor diode structure 102 disposed on a substrate 104, togetherconsidered herein an “LED”, and a phosphor layer or structure 106disposed on the LED. Semiconductor diode structure 102 typicallycomprises an active region disposed between n-type and p-type layers.Application of a suitable forward bias across the diode structureresults in emission of light from the active region. The wavelength ofthe emitted light is determined by the composition and structure of theactive region.

The LED may be, for example, a III-Nitride LED that emits blue, violet,or ultraviolet light. LEDs formed from any other suitable materialsystem and that emit any other suitable wavelength of light may also beused. Other suitable material systems may include, for example,III-Phosphide materials, III-Arsenide materials, and II-VI materials.

Any suitable phosphor materials may be used, depending on the desiredoptical output from the pcLED.

FIGS. 2A-2B show, respectively, cross-sectional and top views of anarray 200 of pcLEDs 100 including phosphor pixels 106 disposed on asubstrate 202. Such an array may include any suitable number of pcLEDsarranged in any suitable manner. In the illustrated example the array isdepicted as formed monolithically on a shared substrate, butalternatively an array of pcLEDs may be formed from separate individualpcLEDs. Substrate 202 may optionally comprise CMOS circuitry for drivingthe LED, and may be formed from any suitable materials.

As shown in FIGS. 3A-3B, a pcLED array 200 may be mounted on anelectronics board 300 comprising a power and control module 302, asensor module 304, and an LED attach region 306. Power and controlmodule 302 may receive power and control signals from external sourcesand signals from sensor module 304, based on which power and controlmodule 302 controls operation of the LEDs. Sensor module 304 may receivesignals from any suitable sensors, for example from temperature or lightsensors. Alternatively, pcLED array 200 may be mounted on a separateboard (not shown) from the power and control module and the sensormodule.

Individual pcLEDs may optionally incorporate or be arranged incombination with a lens or other optical element located adjacent to ordisposed on the phosphor layer. Such an optical element, examples ofwhich are referred to in greater detail below, may be referred to as a“primary optical element”. In addition, as shown in FIGS. 4A-4B a pcLEDarray 200 (for example, mounted on an electronics board 300) may bearranged in combination with secondary optical elements such aswaveguides, lenses, or both for use in an intended application. In FIG.4A, light emitted by pcLEDs 100 is collected by waveguides 402 anddirected to projection lens 404. Projection lens 404 may be a Fresnellens, for example. This arrangement may be suitable for use, forexample, in automobile headlights. In FIG. 4B, light emitted by pcLEDs100 is collected directly by projection lens 404 without use ofintervening waveguides. This arrangement may particularly be suitablewhen pcLEDs can be spaced sufficiently close to each other, and may alsobe used in automobile headlights as well as in camera flashapplications. A microLED display application may use similar opticalarrangements to those depicted in FIGS. 4A-4B, for example. Generally,any suitable arrangement of optical elements may be used in combinationwith the pcLEDs described herein, depending on the desired application.

As summarized above, this specification discloses pixelated pcLED arraysin which the pcLED pixels each comprise an optical element on the lightemitting surface above the phosphor pixel, formed from a retainedportion of a sacrificial phosphor carrier substrate used during themanufacturing process. Much or most of the sacrificial phosphor carriersubstrate is removed during the manufacturing process.

The partial cross-sectional views of FIGS. 5A-5F schematically showstages in an example method for fabricating such a pixelated pcLEDarray. In FIG. 5A, a phosphor layer 500 is deposited on a phosphorcarrier substrate 505. Phosphor layer 500 may comprise phosphorparticles dispersed in a binder such as a silicone binder, for example.In such cases the binder may be partially or fully cured at this stage.Any suitable phosphor materials may be used. Suitable phosphor materialsmay include, for example, YAG, LuAG, silicates, BOSE, β-SiAlON, SCASN,BSSN, KSiF:Mn, SLA, and quantum dots. Phosphor layer 500 may have athickness of about 20 microns to about 500 microns, for example.

Phosphor carrier substrate 505 may be formed, for example, from atransparent glass, a tinted (colored) glass, a crystalline material, atranslucent material, a polarizing material, a polarization rotatingmaterial, or a material that functions as a dichroic filter. If theresulting array is to be used as a mobile phone flash, having atranslucent layer on top of an otherwise vivid-looking phosphor maydesired by the user. Optionally, phosphor carrier 505 may be formed froma material permeable to oxygen, allowing oxygen to diffuse into andthrough the phosphor pixel in the completed pcLED pixels. Suitableoxygen permeable materials may include, for example, porous materialssuch as porous glasses, perforated glasses, and glass-fiber reinforcedplastics.

Phosphor carrier substrate 505 may have a thickness of about 50 micronsto about 200 microns, typically about 100 microns, for example. Anysuitable thickness for phosphor carrier 505 may be used.

The surface of phosphor carrier substrate 505 on which the phosphorlayer is deposited may be pre-patterned to provide a desired opticaleffect for the resulting pcLED pixels. For example, the pre-patterningmay form a lens (e.g., a Fresnel lens) for each of the resulting pcLEDpixels, provide features (e.g., grooves, ridges, protrusions, or othertexturing features) that enhance light extraction from the pcLED pixelsinto air, or scatter light to provide an off-state white diffuser layerfor each pcLED pixel.

Subsequently, as shown in FIG. 5B, individual phosphor pixels aredefined (singulated) by forming trenches 520 through phosphor layer 500and into, but not entirely through, phosphor carrier substrate 505 toform an array of phosphor pixels 515. This may be done by sawing, forexample, but any suitable singulation method may be used. The phosphorpixels may be formed in a rectangular (e.g., square) array, for example,by intersecting trenches formed in phosphor layer 500.

Trenches 520 may have a width of, for example, about 5 microns to about200 microns. Trenches 520 may penetrate phosphor carrier substrate 505to a depth of, for example, about one quarter or more of the thicknessof the substrate but must be shallower than the thickness of thesubstrate to keep the pixels unseparated. Trenches 520 may be spacedapart from each other by, for example, about 5 microns to about 200microns.

Subsequently, as shown in FIG. 5C, each phosphor pixel 515 in the arrayof phosphor pixels is attached to a corresponding LED 525 in an array ofsemiconductor LEDs. LEDs 525 may be supported by a substrate 530, forexample, which may be formed from any suitable material. Attachment ofthe phosphor pixels to the LEDs may be by any suitable method. If thephosphor pixels comprise phosphor particles dispersed in a binder, theymay for example be attached to the individual LEDs by curing or furthercuring the binder to form bonds to the LEDs. Alternatively, or inaddition, a separate adhesive layer may be used to attach the phosphorpixels to the LEDs.

Subsequently, as shown in FIG. 5D, a sufficient thickness of phosphorcarrier 505 is removed so that phosphor pixels 515 are no longerinterconnected by the phosphor carrier and side walls 535 of thephosphor pixels and of the LEDs are exposed for side coating, describednext. Each individual phosphor pixels retains a portion 537 of phosphorcarrier 505 on its upper light output surface. For example, a thicknessof about 50 microns to about 200 microns of phosphor carrier 505 may beremoved at this step. Retained portion 537 of phosphor carrier 505 mayhave a thickness of, for example, about 5 microns to about 50 microns.

Subsequently, as shown in FIG. 5E, a reflective or scattering material540 is deposited on the top and sides of the phosphor pixels and thesides of the LEDs. Reflective or scattering material 540 may be orcomprise, for example, a light scattering material such as TiO₂particles embedded in silicone, one or more reflective metal layers, orone or more DBR structures formed from a stack of alternating layers ofhigh and low refractive index material. Reflective metal layers may bedeposited by vapor deposition or sputtering, for example. DBR structuresmay be deposited by atomic layer deposition, for example.

Subsequently, as shown in FIG. 5F, excess reflective or scatteringmaterial 540 is removed from the upper light output surfaces of thephosphor pixels by any suitable method, for example by mechanicalgrinding or polishing. Optionally, a further portion of phosphor carrier505 (for example, about 1 micron to about 45 microns) is removed alongwith the excess portion of material or structure 540. As noted above, aportion 545 of phosphor carrier 505 is retained on the upper lightoutput surface of each phosphor pixel. Retained portion 545 may have athickness of, for example, about 5 microns to about 10 microns, or anyother suitable thickness.

Remaining portions of reflective or scattering material 540 form sidewalls 550 on the phosphor pixels and LEDs, optically isolatingindividual pcLED pixels from each other.

Depending on the intended application for the pcLEDs and theirdimensions, the resulting pixelated array shown in FIG. 5F may bemaintained essentially intact, transferred as an array to anothersubstrate, or divided into separate pcLEDs.

Retaining phosphor carrier portions 545 on the top output surface of thephosphor pixels relaxes manufacturing tolerances compared to a processin which all of sacrificial phosphor carrier 505 is removed to exposethe top of the phosphor pixels. This is because variations in thethickness of retained phosphor carrier portions 545 typically havelittle effect on the performance of the pcLED pixels. In contrast,variations in the thickness of the phosphor pixels may significantlyaffect the output of the pcLED pixels, because as explained above theamount of light emitted by the LEDs that is absorbed and converted tolonger wavelength light by the phosphor pixels depends on the pathlength of the LED light through the phosphor pixels and thus on theheight of the phosphor pixels. Consequently, if all of the sacrificialphosphor carrier 105 is removed, the end point of the removal processmust be controlled with sufficient precision so as not to affect thephosphor pixels. That may be challenging.

Further, in a process in which all of sacrificial phosphor carrier 505is removed, the finish surface of the phosphor pixels cannot be easilycontrolled to achieve a desired effect such as, for example, affectinglight extraction or output radiation pattern.

As explained above, retaining phosphor carrier portion 545 on the topoutput surface of the phosphor pixel allows incorporating a primaryoptic into the pcLED pixel during the manufacturing process for thepcLED. This may be advantageous compared to a process in which such aprimary optic is separately prepared and subsequently attached to apcLED pixel. For example, subsequently attaching a primary optic to apcLED pixel may require additional process steps, may require use of anadhesive or glue that can spill onto other portions of the pcLED pixel,and would require aligning the primary optic with the pcLED pixel. Usingretained phosphor carrier portion 545 as a primary optic avoids thesedifficulties.

Depending on the viscosity of the phosphor material, the retainedphosphor carrier portion 545 (the optical element) may retain airstructures between the optical element and the film and thereby enhancerefractive index contrast and maximize optical performance.

This disclosure is illustrative and not limiting. Further modificationswill be apparent to one skilled in the art in light of this disclosureand are intended to fall within the scope of the appended claims.

What is claimed is:
 1. A method of fabricating an array ofphosphor-converted LEDs, the method comprising: forming a phosphor layeron a first surface of a carrier substrate; forming trenches extendingentirely through the phosphor layer and partially but not entirelythrough the carrier substrate to define a plurality of spaced-apartphosphor pixels; attaching a corresponding LED to each of the phosphorpixels to form an array of phosphor-converted LEDs arranged on the firstsurface of the carrier substrate; removing a sufficient portion of thecarrier substrate from a second surface of the carrier substrate,opposite from the first surface, to reach and open the trenches andthereby expose phosphor pixel side walls defined by the trenches; andafter removing the sufficient portion of the carrier substrate to reachand open the trenches, retaining a portion of the carrier substrate on alight-output surface of each phosphor pixel.
 2. The method of claim 1,wherein the first surface of the carrier substrate is patterned so thatthe retained portion of the carrier substrate on the light-outputsurface of each phosphor pixel is a lens.
 3. The method of claim 1,wherein the first surface of the carrier substrate is patterned so thatthe retained portion of the carrier substrate on the light-outputsurface of each phosphor pixel enhances light extraction from thephosphor pixel.
 4. The method of claim 1, wherein the first surface ofthe carrier substrate is patterned so that the retained portion of thecarrier substrate on the light-output surface of each phosphor pixelscatters light so that the light-output surface appears white underwhite light illumination when the phosphor-converted LED is notoperating.
 5. The method of claim 1, wherein the carrier substrate isformed from a glass that is substantially transparent at outputwavelengths of the phosphor-converted LEDs.
 6. The method of claim 1,wherein the carrier substrate is formed from a colored glass.
 7. Themethod of claim 1, wherein the carrier substrate is formed from amaterial that is translucent under white light illumination.
 8. Themethod of claim 1, wherein the carrier substrate is formed from acrystalline material.
 9. The method of claim 1, wherein the retainedportion of the carrier substrate is a dichroic filter.
 10. The method ofclaim 1, wherein the retained portion of the carrier substrate ispermeable to oxygen.
 11. The method of claim 1, wherein the retainedportion of the carrier substrate polarizes light.
 12. The method ofclaim 1, wherein the retained portion of the carrier substrate rotatesthe polarization of light.
 13. The method of claim 1, comprising afterexposing the side walls of the phosphor pixels, depositing lightscattering or reflective material on the side walls of the phosphorpixels.
 14. The method of claim 13, comprising after depositing lightscattering or reflective material on the side walls of the phosphorpixels, removing scattering or reflective material from a light-outputsurface of the retained portion of the carrier substrate on eachphosphor pixel.
 15. The method of claim 14, comprising after removingscattering or reflective material from the light-output surface of theretained portion of the carrier substrate on each phosphor pixel;removing some but not all of the retained portion of the carriersubstrate from each phosphor pixel.
 16. The method of claim 13, whereinthe first surface of the carrier substrate is patterned so that theretained portion of the carrier substrate on the light-output surface ofeach phosphor pixel is a lens.
 17. The method of claim 13, wherein thefirst surface of the carrier substrate is patterned so that the retainedportion of the carrier substrate on the light-output surface of eachphosphor pixel scatters light so that the light-output surface appearswhite under white light illumination when the phosphor-converted LED isnot operating.
 18. The method of claim 13, wherein the carrier substrateis formed from a material that is translucent under white lightillumination.
 19. The method of claim 13, wherein the retained portionof the carrier substrate is porous and permeable to oxygen.